The Brumbies' appointment of Newcastle Knights veteran Danny Buderus as a part-time coach is over even before it started.

The NRL has blocked the Super Rugby franchise's move to sign Buderus as a collision coach, saying no third party agreement has been lodged as the salary cap requires.

The league issued a statement seeking "an immediate explanation" from the Knights with regards to the deal.

"It is hard to envisage any circumstance where an agreement could be approved that allowed a current NRL player to promote another code competing in the same market as an NRL team," interim NRL chief executive Shane Mattiske said.

"The announcement today which saw Danny at a press conference in another code's training gear underlines potential issues.

"...as much as we respect Danny’s intentions we do not see any way in which the proposal can be pursued."

"We will have to look for someone else but we don't have anyone in mind at the moment."

Fagan said he thought that Buderus had the Knights' blessing but evidently not the NRL's.

"It's certainly an unfortunate series of events," he said earlier on Thursday.

"We tried to provide Danny with an opportunity to pursue some coaching, which he understood he was keen to do.

"That was endorsed and supported by the Knights organisation and Wayne Bennett but unfortunately the NRL weren't aware of the deal.

"They've got some issues with it we would like to see those issues worked through but if that is not the case then we just have to move on."

Fagan said that because Newcastle had endorsed the deal, he assumed the NRL had as well.

The Knights have since said in a statement that the final agreement between Buderus and the Brumbies had not been seen by the club and needs to be submitted to and approved by the NRL.

But Newcastle chief executive Matt Gidley did say the club had encouraged Buderus to seek varied work experience.

"As Danny enters his final NRL season before retirement, we have encouraged him, as we do with all our players, to engage in wide ranging work experience in preparation for life after football," Gidley said.

Mattiske said the NRL provides avenues for players to expand their scope for post-football careers.

"In conjunction with the RLPA [Rugby League Players Association] we provide a range of specialist services to assist players in maximising off-field careers," he said.